Improve handling of dictionary conversions.

Source/core/events/MessageEvent.h

Index: Source/core/events/MessageEvent.h
diff --git a/Source/core/events/MessageEvent.h b/Source/core/events/MessageEvent.h
index 2a73181a7ed2ef207869d7200dd32c7b9fe01d16..64df40f771891b8cd4ff2a16ccb14205d92240e0 100644
--- a/Source/core/events/MessageEvent.h
+++ b/Source/core/events/MessageEvent.h
@@ -77,10 +77,7 @@ public:
     {
         return adoptRef(new MessageEvent(data, origin));
     }
-    static PassRefPtr<MessageEvent> create(const AtomicString& type, const MessageEventInit& initializer)
-    {
-        return adoptRef(new MessageEvent(type, initializer));
-    }
+    static PassRefPtr<MessageEvent> create(const AtomicString& type, const MessageEventInit& initializer, ExceptionState&);
     virtual ~MessageEvent();
 
     void initMessageEvent(const AtomicString& type, bool canBubble, bool cancelable, const String& origin, const String& lastEventId, DOMWindow* source, PassOwnPtr<MessagePortArray>);
@@ -89,6 +86,7 @@ public:
     const String& origin() const { return m_origin; }
     const String& lastEventId() const { return m_lastEventId; }
     EventTarget* source() const { return m_source.get(); }
+    EventTarget* source(bool& isNull) const { isNull = !m_source; return m_source.get(); }
     MessagePortArray ports() const { return m_ports ? *m_ports : MessagePortArray(); }
     MessagePortChannelArray* channels() const { return m_channels ? m_channels.get() : 0; }